Semiconductor device microstructure

ABSTRACT

A semiconductor device comprising a semiconductor body having a depression formed into the first surface of the body. The device further comprises member means comprising first and second thermal-to-electric transducer or static electric element, the member means having a predetermined configuration suspended over the depression. The member means is connected to the first surface at least at one location, the depression opening to the first surface around at least a portion of the predetermined configuration. The depression provides substantial physical and thermal isolation between the elements and the semiconductor body. In this manner, an integrated semiconductor device provides an environment of substantial physical and thermal isolation between the transducer or element and the semiconductor body.

This application is a continuation of application Ser. No. 480,644,filed Mar. 31, 1983, now abandoned, which is a division of applicationSer. No. 310,262, filed Oct. 9, 1981, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to integrated semiconductor devicetechnology in the field of sensors and radiant sources ofelectromagnetic energy. More specifically, the present inventioncomprises an integrated semiconductor device which provides a new microenvironment for applications including sensing. The presentsemiconductor device, which may be fabricated through batch processing,provides an environment which permits a thermal-to-electric transduceror static electric elemet to be integrated with a semiconductor circuitchip while having substantially greater thermal and physical isolationfrom the chip than is possible with conventional emplacements of suchcomponents in integrated semiconductor devices. The present inventionhas applications in areas including flow sensing, detection ofcombustible gases, humidity sensing, and pressure sensing. However, thepresent invention is not limited to such applications.

The present invention comprises a semiconductor device and a method forfabricating the semiconductor device.

The semiconductor device comprises a semiconductor body having adepression formed into the first surface of the body. The device furthercomprises member means comprising a thermal-to-electric transducer orstatic electric element, the member means having a predeterminedconfiguration suspended over the depression. The member means isconnected to the first surface at least at one location, the depressionopening to the first surface around at least a portion of thepredetermined configuration, the depression providing substantialphysical and thermal isolation between the element and the semiconductorbody.

In this manner, an integrated semiconductor device provides anenvironment of substantial physical and thermal isolation between thetransducer or element and the semiconductor body.

The method of making such a device comprises the steps of providing asemiconductor body with a first surface having a predeterminedorientation with respect to a crystalline structure in the semiconductorbody. The method further comprises applying a layer of material of whichthe member means is comprised onto the first surface. The method alsocomprises exposing at least one predetermined area of the first surface,the exposed surface are being bounded in part by the predeterminedconfiguration to be suspended, the predetermined configuration beingoriented so that undercutting of the predetermined configuration by ananisotropic etch will occur in a substantially minimum time. Finally,the method comprises applying the anisotropic etch to the exposedsurface area to undercut the member means and create the depression.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 3 are cross-sectional drawings of preferred embodimentsin accordance with the present invention.

FIG. 4 illustrates an electrically resistive grid compatible with thepresent invention.

FIG. 5 illustrates circuitry compatible with some preferred embodimentsof the present sensor.

FIGS. 6-8 illustrate a sensor in accordance with the present invention.

FIG. 9 illustrates a combustible gas sensor in accordance with thepresent invention.

FIGS. 10-13 illustrate orientation and alternate preferred embodimentsof microstructure detail compatible with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As previously indicated, the present inventions have applications inareas such as flow sensing, detection of combustible gases, humiditysensing, and pressure sensing. These specific applications will bediscussed below, followed by discussions of generic devices andprocessing steps associated with fabrication of the present structures.

Flow Sensor

Thermal anemometry has been for many years a useful tool for themeasurement of fluid flow. Thermal anemometers, by definition, dependupon heat transfer for their operation. A resistor with a temperaturesensitive resistance is typically placed in the flow stream. Electriccurrent flowing through the resistor causes the resistor to increase intemperature due to electrical power dissipation. The fluid beingmonitored carries heat away from the resistor via forced convection. Theultimate temperature of the resistor, as indicated by measuredresistance, is a function of the fluid's velocity and thermalconductivity. Prior art transducing resistive elements are typically ofthe hot-wire, hot-film, thermistor type.

The ideal thermal anemometer would be inexpensive yet possess a veryfast response resistor/transducer and be accurate and rugged. Theserequirements are often conflicting, as evidenced by state-of-the-artthermal anemometers. Cheap anemometers are typically comprised of bulkysensing elements which causes a poor response time characteristic. Fastresponding aneometers are typically expensive and have fragile sensingelements. Accurate anemometers are typically expensive due to laborintensive assembly of the sensing element and support structure.Moreover, state-of-the-art anemometers must be fully inserted into thefluid flow region and, consequently, are subject to destruction ordeterioration by impacting dust, lint, or other debris.

The present thermal anemometer or flow transducer more closely providesall the features desired in an ideal transducer. The disclosed device isinexpensive, as it can be fabricated by low cost batch processes such assilicon-compatible processes; the device responds with a thermal timeconstant in the millisecond range; and the accuracy of the deviceexceeds those of prior solid state thermal anemometers due to anincreased sensitivity (greater change in resistance for a given changein flow) and signal to noise ratio. Moreover, its design is such that itneed not be fully inserted into the fluid stream. As a result, dust,lint, and debris tend to flow by the sensing element rather thanimpacting it. The present anemometer is therefore less subject toperformance deterioration than state-of-the-art thermal anemometers.

FIGS. 1 and 2 are cross-sectional side views of alternate preferredembodiments of flow sensors in accordance with the present invention. Amono-crystalline semiconductor 10 has a first surface 14 covered with adielectric layer 12 such as silicon nitride. In the embodiment shown, anelement 22 (FIG. 4) comprises a permalloy resistive element or grid 16and leads 24 sputtered onto dielectric layer 12, element 22 beingcovered with a layer 18 of dielectric such as silicon nitride.

Dielectric layer 12 provides electrical isolation between element 22 andsemiconductor 10 (layers 12 and 18 also provide passivation for element22). Substantial thermal and physical isolation is provided betweenresistive element 16 and semiconductor 10 by forming a depression 20below element 16. Depression 20 is typically formed using preferentialetching techniques such as those discussed elsewhere herein. Withoutdepression 20, it would be difficult to achieve substantial thermal andphysical isolation between sensing element 16 and semiconductor 10; forexample, if resistive element 16 were separated from semiconductor 10 byonly a solid dielectric layer, resistive element 16 would besubstantially heat-sunk into semiconductor 10 since the thermalconductivities of solid dielectrics are typically much greater than thethermal conductivity of air.

Substantial thermal and physical isolation between sensing element 16and semiconductor 10 provides many advantages adaptable to a widevariety of devices such as sensors. For example, in the case of thepresent semiconductor body flow sensor, by providing a very thin sensingmeans that is substantially thermally isolated from the semiconductorbody, the sensing means is adapted to provide a very sensitivemeasurement of air flow, since the temperature of the thin structurewill be readily affected by the air flow. This is in contrast to solidstate thermal anemometers that have sensing elements substantially heatsunk into the semiconductor body; the temperature sensitivity of suchstructures is greatly affected by the thermal mass of the semiconductoritself.

In the embodiment of FIG. 1, member or sensing means 34 is bridgedacross depression 20, member means 34 having first and second ends 38and 40 connected to semiconductor first surface 14. As disclosed, membermeans 34 is substantially rectangular in shape when viewed from above,member means 34 as disclosed comprising resistive element 16 and aportion of dielectric layers 12 and 18.

In the embodiment of FIG. 2, member or sensing means 32, comprising aresistive element 16 and a portion of dielectric layers 12 and 18, iscantilevered over depression 20, only one end 36 of member means 32being connected to semiconductor first surface 34. Having only one endof member means 32 connected to semiconductor body 10 provides certainadvantages, including the advantage of permitting member means 32 toexpand and contract in substantially all directions without substantialrestraint from semiconductor body 10. In addition, member means 32provides an embodiment having substantially increased thermal isolationbecause heat loss by conduction through member means 32 occurs throughonly one supporting end.

FIG. 3 illustrates an end cross-sectional view of a preferred embodimentcomprising two member means 32 or 34 (FIGS. 10-13 illustrate top viewsof various alternate preferred embodiments). With regard to the presentflow sensor, a pair of member means is a preferred embodiment havingcertain advantages. For example, as is further discussed below, usingtwo substantially identical member means and offsetting a signal fromone against a signal from the other can provide automatic temperaturecompensation for changing environmental temperatures. In addition, suchan arrangement can greatly increase measurement accuracy, sincebackground voltage within a single sensor can easily be substantiallyeliminated. Further, using two measurement elements in a flow sensorprovides an indication of flow direction as well as rate, since theupstream element cools more than the downstream element, as is furtherexplained below.

However, a single sensing means supported over depression 20 isconsistent with the present flow sensor; for example, for making aflow/no-flow measurement, an air turbulence signal generated in asingle-sensing-element flow sensor would be adequate to establish thepresence or the absence of air flow. Amplification of only the a.c.(turbulent flow) component of the element resistance change eliminatesdetection of slow or d.c. changes in element resistance caused, forexample, by changing environmental temperature.

For the preferred embodiments shown, permalloy was selected to formresistive element 16 because permalloy can be precisely deposited bysputtering a layer only hundreds of angstroms thick, and because thecharacteristics of permalloy provide a highly sensitive predeterminedrelationship between the resistance of grid or element 16 and thetemperature of the element. For example, a very thin member or sensingmeans 32 or 34 may be formed of resistive element 16 and dielectriclayers 12 and 18. When applied as a flow sensor, air flowing over membermeans 32 or 34 will cause resistive element 16 to cool in predeterminedrelation to the rate of air flow, thus causing a change in resistanceand providing a measurement of the air flow.

For the embodiments shown, member means 32 and 34 are typically on theorder of 0.8-1.2 microns thick, this thickness including element 16(typically on the order of 800 angstroms thick) and dielectric layers 12and 18, each typically on the order of thousands of angstroms thick.This very thin and highly sensitive configuration, in combination withthe fact that sensing element 16 is substantially isolated from the bodyof the semiconductor 10 by depression 20 (typically in the range of0.001 to 0.010 inch deep) causes the sensing means to be highlysensitive to flow measurements.

As previously indicated, a preferred embodiment of element 16 comprisesa permalloy grid as shown in FIG. 4. Leads 24 may be permalloy sinceadditional processing steps are then avoided (making leads 24 of anothermaterial would require additional processing steps; although leads 24 ofpermalloy become slightly heated, they are relatively wide, asillustrated in FIGS. 4 and 10-13, and they are substantially heat sunkinto semiconductor body 10, making heating of leads 24 relativelyminor).

As previously indicated, advantages exist in providing a flow transducercomprising first and second resistive elements such as illustrated inFIG. 3. Such an embodiment may be coupled with a circuit such asillustrated in FIG. 5, thus providing a flow transducer which isindependent of environmental temperatures and which provides furthersensitivity by eliminating background signals and directly providing ameasurement signal.

For the purpose of discussing the operation of the sensor embodimentillustrated in FIG. 3 and for the purpose of describing the circuitillustrated in FIG. 5, the sensing elements in these figures have beenlabeled 16A and 16B, each element 16A and 16B comprising an element 16.Elements 16A and 16B are typically matched (at least substantiallyidentical), but they do not have to be matched.

A substantial advantage of the present invention is that circuits suchas illustrated in FIG. 5 may be integrated directly onto semiconductorbody 10, thus providing a complete sensing system on a single chipthrough batch processing.

The circuit illustrated in FIG. 5 comprises three differentialamplifiers, each of which may comprise, for example, a TLO87. Asillustrated, each of two amplifiers 50 and 52 have a resistive element16A or 16B connected across its feedback loop. Thus, resistive element16A is connected between an output 54 and a negative input 59 ofamplifier 50, the connections being made via leads 24 associated withelement 16A. Resistive element 16B is connected across an output 56 anda negative input 58 of amplifier 52, the connections again being madewith leads 24 associated with element 16B.

Negative input 58 to amplifier 52 is connected to a wiper 66 of apotentiometer 62 through a resistor 64. Negative input 59 to amplifier50 is connected to wiper 66 through a resistor 70. Positive inputs 72and 74 of amplifiers 50 and 52 respectively are connected to ground orreference potential 76.

Output 56 of amplifier 52 is connected to a negative input 78 ofamplifier 80 through a resistor 82, output 54 of amplifier 50 beingconnected to a positive input 84 to amplifier 80 through a resistor 86.Positive input 84 of amplifier 80 is also connected to ground orreference potential 76 through a resistor 88. A resistor 90 is connectedbetween an output 92 of amplifier 80 and negative input 78 of theamplifier.

A first terminal 94 of potentiometer 62 is provided for connection to apositive power supply such as +15 VDC, a second terminal 96 ofpotentiometer 62 being provided for connection to a negative powersupply such as -15 VDC. Potentiometer 62 provides a means for selectinga predetermined voltage anywhere between the plus and minus voltage ofthe power supply.

In operation, the disclosed circuit generates a voltage between output92 and ground or reference potential 76 that bears a predeterminedrelationship to gas flow rate over member means 32 or 34 comprisingresistive elements 16A and 16B.

Resistive elements 16A and 16B are configured into the feedback loops ofamplifiers 50 and 52, respectively. Each operational amplifier 50 and 52maintains constant current through its feedback loop. Therefore, theelectrical current flowing through each resistive element 16A and 16B isindependent of the element resistance. In order to maintain constantcurrent in its feedback loop, each operational amplifier in effectchanges its output voltage in proportion to the change in resistance ofresistive elements 16A or 16B. As previously indicated, the resistanceof each permalloy element 16A or 16B changes in a predetermined mannerwith the temperature of the resistive element. Therefore, the voltageoutput of each operational amplifier 50 and 52 bears a predeterminedrelationship to the associated resistive element temperature.

Operational amplifier 80 amplifies the difference between the voltageoutputs of operational amplifiers 50 and 52, the voltage at output 92being proportional to the difference in voltage between the outputvoltages of operational amplifiers 50 and 52. Accordingly, the voltageat output 92 bears a predetermined relationship to the temperaturedifference between resistive elements 16A and 16B. The temperaturedifference between resistive elements 16A and 16B bears a predeterminedrelationship to the gas flow rate over the elements. Therefore, thevoltage at output 92 of amplifier 80 bears a predetermined relationshipto the rate of flow across elements 16A and 16B.

Gas flowing first over one member or sensing means comprising resistiveelement 16A and then over a second member or sensing means comprisingelement 16B causes resistive element 16A to cool more than resistiveelement 16B, since the gas flowing over element 16A picks up heat fromelement 16A and carries heat to the vicinity of element 16B. Assumingthat the circuit supply voltage at wiper 66 is positive, this causes theoutput voltage of amplifier 52 to be greater than the output voltage ofamplifier 50. This difference is magnified by amplifier 80, the outputvoltage at output 92 bearing a predetermined relationship to the rate ofgas flow. As previously indicated, the output voltage at output 92 canalso provide an indication of directionality. For example, if elements16A and 16B are aligned along the flow in a duct, the presenttwo-element sensor can be used to determine direction of flow as well asflow rate, since the upstream element will cool more than the downstreamelement, as explained above.

As indicated, the circuit illustrated in FIG. 5 operates sensingelements 16A and 16B in a constant current mode. It should be noted thatother circuits are also possible, including circuits which would operatesensors 16A and 16B (or any of the present sensors) in a constantvoltage mode, a constant temperature (constant resistance) mode, or aconstant power mode.

Humidity Sensor

The present invention also has applications as a humidity sensor. Insuch applications, the present sensor can measure atmospheric watervapor concentration or relative humidity independently of surfaceadsorption effects and independently of optical effects and does this atvery low cost on a semiconductor chip compatible with signal processingintegration.

The present humidity sensor depends upon the variation of thermalconductivity of air as the water vapor concentration varies. Water vaporconcentration here is defined as the ratio of the number of water vapormolcules per unit volume to the number of molecules of dry air per unitvolume. This concentration is sometimes referred to as molal humiditywhich is related to specific humidity by the constant ratio of themolecular weight of water to the average molecular weight of dry air.

Accordingly, the present humidity sensor provides a direct indication ofmolal humidity which may be converted to specific humidity through acircuit (not shown) providing an appropriate multiplier to the molalhumidity measurement.

It may also be of interest to convert molal humidity measurements tomeasurements of relative humidity. Such a conversion requires ameasurement of ambient temperature and requires that a correspondingautomatic adjustment be made according to standard psychrometric chartdata. An appreciable altitude effect caused by air mixture densityvariation is also involved in a conversion to relative humidity because,for any given mole fraction of water vapor measured by thermalconductivity, the partial pressure of the water vapor will vary withaltitude. Consequently, for the most accurate relative humiditymeasurement, the conversion must be adjusted slightly by analtitude-dependent factor. Such conversions may be made by circuitry(not shown).

Environmental control applications may require a device readout in termsof air mixture enthalpy referred to the enthalpy at some lower referencetemperature and zero humidity. Enthalpy varies linearly with temperatureat constant molal humidity and linearly with molal humidity at constanttemperature in ranges of interest that include freezing andcondensation. Therefore, an enthalpy determination can be obtained froma molal humidity measurement and the air mixture temperature by acircuit (not shown) that yields a readout offset for dry airproportional to the difference between mixture temperature and referencetemperature, and which calibrates the molal humidity output to anenthalpy scale.

In its simplest form, the present humidity sensor comprises asemiconductor body 100 have a depression 102 etched or otherwise formedinto a first surface 104 of the body. The present humidity sensorfurther comprises member means such as 106 which may be either bridgedacross depression 102 in the manner of member means 34 shown in FIG. 1,or may be of a cantilever configuration as shown in FIGS. 2 and 6.Member means 106 typically comprises a resistive element such as 108,member means 106 having a predetermined configuration suspended over thedepression, the member means being connected to first surface 104 atleast at one location such as location 110. Depression 102 opens tofirst surface 104 around at least a portion of the predeterminedconfiguration of member means 106.

Resistive element 108, when provided with current, becomes heated, therebeing a predetermined relationship between the resistance and thetemperature of resistive element 108.

The present humidity sensor further comprises no-flow means 116, FIG. 8,for substantially preventing air flow over member means 106, thussubstantially preventing cooling of resistive element 108 due to airflow. The no-flow means permits the humidity level to equalize betweenmember means 106 and semiconductor body 100 with the humidity level inthe surrounding environment, the no-flow means example 116 containingapertures 118 for this purpose. Also shown are filters 120 to preventthe present sensor from becoming contaminated by air-borne particulates.

Resistive element 108 is adapted to provide a signal having a magnituderelated to the resistance and, therefore, the temperature of element108, the magnitude of the signal varying with humidity due to varyingthermal coupling between element 108 and semiconductor body 100 throughdepression 102, the varying thermal coupling arising through the varyingconductivity of air with varying molal humidity, the signal thusproviding a measurement of humidity.

In a typical application of the present humidity sensor, semiconductorbody or chip 100 is mounted with epoxy to a glass member 114, which inturn is mounted to a header 112. The glass member substantiallythermally isolates substrate 100 from the header. The header typicallycomprises feedthroughs (not shown) in order to connect a wire bondedstructure for making electrical connections.

The present humidity sensor may also comprise reference resistor means112 comprising a resistive element 124. As further discussed below, thepresent humidity sensor may further comprise series resistor means 126having a substantially zero thermal coefficient of resistance (TCR) overa predetermined temperature range of interest. As shown in FIG. 7 andexplained below, series resistor means 126 may be connected in serieswith resistive element 124; alternately, series resistor means 126 maybe connected in series with resistive element 108. As examples, seriesresistor means 126 may comprise a chrome silicide or a nichrome element.

The present humidity sensor may further comprise heater means comprisingan element 128 for controlling the temperature of semiconductor body 100to a predetermined temperature. Element 128 may comprise a resistiveelement such as a permalloy element substantially heat sunk to thesemiconductor body.

Like resistive element 108, resistive element 124 may comprise apermalloy grid not unlike that shown in FIG. 4. As such, element 124 mayserve not only as a reference resistor for element 108 but also as atemperature measuring sensor for element 124 and/or for thermostatedsemiconductor body 100, the parmalloy having a predeterminedrelationship between temperature and resistance. Thus, semiconductorbody 100 can be maintained at a predetermined elevated temperature byregulating current through resistive element 128 and monitoring thetemperature of body 100 with element 124.

As disclosed, elements 108, 124, 126, and 128 are sandwiched between twolayers of dielectric such as silicon nitride, a first layer 127 alsocovering at least a portion of first surface 104, and a second layer129.

With a permalloy element 124 substantially heat sunk to semiconductorbody 100, the temperature of element 124 is substantially regulated bythe temperature of the semiconductor body. Further, since element 124 issubstantially thermally coupled to semiconductor body 100, theresistance of element 124 does not substantially vary with varyinghumidity. Accordingly, a signal from element 124 may be offset by asignal from element 108, thus effectively providing a resulting signalthat under predetermined conditions of specific humidity will have apredetermined value. A circuit substantially as shown in FIG. 5 may beused to attain such a result, elements 16A and 16B of FIG. 5 beingreplaced with elements 108 and 124 of the humidity sensor, and element126 being put in series with either element 108 or 124 as appropriate.

The temperature versus resistance curves of permalloy elements arenonlinear. Thus, the temperature versus resistance curve of element 108will have a first predetermined slope when operating at a firstpredetermined operating temperature. The resistance of element 124 maythen be established so that, at a second predetermined temperature,e.g., typically the thermostated temperature of chip or body 100 asmeasured by element 124, the temperature versus resistance curve ofelement 124 has a slope substantially equal to the first predeterminedslope of element 108 at its operating temperature. The total effectiveresistance of element 108 or 124, as appropriate, may then be adjustedby adding series resistor means 126 in series with either element 108(this arrangement is not shown) or 124 (as shown), series resistor means126 having a substantially zero thermal coefficient of resistance overthe temperature range of interest. As a result, the total effectiveresistance of the reference element may be made to be substantiallyequal at the second predetermined temperature to the total effectiveresistance of the humidity sensing element at the first predeterminedtemperature. In this manner, when the effective resistance of thereference and humidity sensing elements are substantially equal, signalsthrough the two elements may be offset so that at a predeterminedhumidity the sum of the signals will be substantially zero. Again, thiscan be accomplished with a circuit substantially as that shown in FIG.5.

COMBUSTIBLE GAS SENSOR

As previously indicated, the present invention also has applications asa sensor for detecting combustible gas. The present combustible gassensor as disclosed in FIG. 9 is quite similar to the flow sensor asdisclosed in FIG. 3 except that a reactive means 130 is thermallycoupled to one of the resistive elements. When heated in the presence ofa combustible gas and oxygen, reactive means 130 provides an indicationof the presence of a combustible gas. In addition, a no-flow means suchas no-flow means 116 used with the present humidity sensor is also usedto substantially prevent air flow over the first and second member meansof the embodiment shown.

In FIG. 9, reactive means 130 is shown thermally coupled to resistiveelement 142 within member means 140. In one preferred embodiment of thepresent combustible gas sensor, reactive means 130 typically comprises acatalytically reactive thin film of, for example, iron oxide, platinum,or palladium which is heated by resistive element 142. In such anembodiment, the catalytically reactive thin film, when heated in thepresence of a combustible gas and oxygen, causes an exothermic reactionwhich changes the temperature and, accordingly, the resistance of itscorresponding resistive element 142. Thus, the change in temperature ofresistive element 142 due to the exothermic reaction provides a changein the resistance of the element indicating the presence of acombustible gas.

In an alternate preferred embodiment of the present combustible gassensor, reactive means 130 may comprise a metal oxide resistive elementof, for example, iron oxide or tin oxide which is heated by resistiveelement 142 (the metal oxide resistive element may be configured similarto element 16 shown in FIG. 4). In such an embodiment, the metal oxideresistive element changes in resistance when heated by resistive element142 in the presence of a combustible gas and oxygen, thus indicating thepresence of a combustible gas.

More specifically, the present sensor for detecting combustible gascomprises a semiconductor body 132 having a depression 134 etched orotherwise formed into a first surface 136 of the semiconductor body orchip 132.

The present sensor further comprises member means 140 which may beeither bridged across depression 134 in the manner of member means 34shown in FIG. 1, or may be of a cantilever configuration such as membermeans 32 shown in FIG. 2. The member means typically comprises aresistive element such as 142 which may comprise a permalloy grid suchas shown in FIG. 4. Member means 140 has a predetermined configurationsuspended over depression 134, member means 140 being connected to firstsurface 136 at least at one location. Depression 134 opens to firstsurface 136 around at least a portion of the predetermined configurationof member means 140. Member means 140 provides substantial physical andthermal isolation between element 142 and semiconductor body 132. Aspreviously indicated, member means 140 further comprises reactive means130 thermally coupled to resistive element 142.

Resistive element 142 when provided with current becomes heated, therebeing a predetermined relationship between the resistance and thetemperature of resistive element 142.

As also previously indicated, the present combustible gas sensor alsoemploys no-flow means such as no-flow means 116 shown in FIG. 8, theno-flow means substantially preventing air flow over member means 140,thus substantially preventing cooling of resistive element 142 due toair flow. The no-flow means permits access of combustible gas toreactive means 130 by, for example, apertures such as 118 shown in FIG.8.

As previously indicated, in a first preferred embodiment of the presentcombustible gas sensor, reactive means 130 typically comprises acatalytically reactive thin film. In such an embodiment, reactive means130 when heated by resistive element 142 in the presence of acombustible gas and oxygen causes an exothermic reaction, thus causing achange in temperature and, accordingly, resistance of resistive element142. This change in resistance of resistive element 142 indicates thepresence of a combustible gas. In a second preferred embodiment,reactive means 130 typically comprises a metal oxide resistive element.In such an embodiment, resistive element changes resistance when heatedby resistive element 142 in the presence of a combustible gas andoxygen, thus indicating the presence of a combustible gas.

As disclosed, resistive element 142 is encapsulated in two layers ofdielectric such as silicon nitride, a first layer 144 also covering atleast a portion of first surface 136, and a second layer 146. As shown,reactive means 130 is deposited above dielectric layer 146 on membermeans 140.

If the first preferred embodiment of the present combustible gas sensoris used, it may be desirable to employ a second resistive element 150embodied in a second member means 148. As disclosed, a second membermeans 148 has a predetermined configuration suspended over depression134, second member means 148 being connected to first surface 136 atleast at one location, depression 134 opening at first surface 136around at least a portion of the predetermined configuration of membermeans 148. Depression 134 provides substantial physical and thermalisolation between second resistive element 148 and semiconductor body132.

Except that member means 148 is without reactive means such as 130,member means 140 and 148 may be substantially identical. Member means148 including its resistive element 150 may then be used as a referenceelement having substantially the same reaction to changes in ambienttemperature as member means 140, thus providing automatic temperaturecompensation. Further, the signal from reference element 150 may beoffset against the signal from sensing element 142, thus substantiallyeliminating background signal levels and obtaining a direct measurementof the signal caused by the change in temperature introduced by reactivemeans 130. Substantially the same circuit as shown in FIG. 5 may be usedto accomplish this, elements 142 and 150 replacing elements 16A and 16Bshown in the Figure.

PRESSURE SENSOR

The present invention also has applications as a pressure sensor,typically as a sensor for measuring sub-atmospheric pressures. A needexists for a pressure sensor covering a relatively wide dynamic range.For example, general industrial processes employing various gases suchas oxygen, argon, nitrogen, and hydrogen at varying temperatures andpressures frequently require the measurement of pressure as part ofprocess control.

Conventional tungsten-heated thermal conductivity pressure sensors inthe sub-atmospheric pressure range are unsatisfactory because of arelatively low dynamic range, high power and voltage requirements,fragility, relatively low sensitivity (because of low thermalcoefficient of resistance) and low life (tungsten readily oxidizes whenthe oxygen partial pressure increases faster than cooling time constantof the heated tungsten). The present pressure sensor eliminates orreduces the severity of these shortcomings.

The present pressure sensor depends upon the variation of thermalconductance of a gas volume. More specifically, as mean free pathlengths become limited by the distance between a member means such as106 and a semiconductor substrate such as 100 below it, thermalconductivity and heat removal rate from the member means decreases withdecreasing gas pressure. This leads to a temperature increase of aresistive element such as 108 in the member means, assuming that theresistive element is being operated with a constant current.

The present pressure sensor may be constructed substantially like thepresent humidity sensor, and the same Figures as used to describe thepresent humidity sensor will also be used to describe the presentpressure sensor.

In its simplest form, the present pressure sensor comprises asemiconductor body 100 having a depression 102 etched or otherwiseformed into a first surface 104 of the body. The present pressure sensorfurther comprises member means such as 106 which may be either bridgedacross depression 102 in the manner of member means 34 shown in FIG. 1,or may be of a cantilever configuration as shown in FIGS. 2 and 6.Member means 106 typically comprises a resistive element 108, membermeans 106 having a predetermined configuration suspended over thedepression, the member means being connected to first surface 104 atleast at one location such as location 110. Depression 102 opens tofirst surface 104 around at least a portion of the predeterminedconfiguration of member means 106.

Resistive element 108, when provided with current, becomes heated, therebeing a predetermined relationship between the resistance and thetemperature of resistive element 108.

The present pressure sensor further comprises no-flow means 116, FIG. 8,for substantially preventing air flow over member means 106, thussubstantially preventing cooling of resistive element 108 due to airflow. The no-flow means permits the pressure level to equalize betweenmember means 106 and semiconductor body 100 with the pressure level inthe surrounding environment, the no-flow means example 116 containingapertures 118 for this purpose. Also shown are filters 120 to preventthe present sensor from becoming contaminated by air-borne particulates.

Resistive element 108 is adapted to provide a signal having a magnituderelated to the resistance and, therefore, the temperature of element108, the magnitude of the signal varying with sub-atmospheric pressuredue to varying thermal coupling between element 108 and semiconductorbody 100 through depression 102, the varying thermal coupling arisingthrough the varying conductivity of air with varying pressure, thesignal thus providing a measurement of pressure.

In a typical application of the present pressure sensor, semiconductorbody or chip 100 is mounted with epoxy to a glass member 114, which inturn is mounted to a header 112. The glass member substantiallythermally isolates substrate 100 from the header. The header typicallycomprises feed throughs (not shown) in order to connect a wire bondedstructure for making electrical connections.

The present pressure sensor may also comprise reference resistor means122 comprising a resistive element 124. As was discussed in associationwith the present humidity sensor, the present pressure sensor mayfurther comprise series resistor means 126 having a substantially zerothermal coefficient of resistance (TCR) over a predetermined temperaturerange of interest. As shown in FIG. 7, series resistor means 126 may beconnected in series with resistive element 124; alternately, seriesresistor means 126 may be connected in series with resistive element108. As examples, series resistor means 126 may comprise a chromesilicide or a nichrome element.

The present pressure sensor may further comprise heater means comprisingan element 128 for controlling the temperature of semiconductor body 100to a predetermined temperature. Element 128 may comprise a resistiveelement such as a permalloy element substantially heat sunk to thesemiconductor body.

Like resistive element 108, resistive element 124 may comprise apermalloy grid not unlike that shown in FIG. 4. As such, element 124 mayserve not only as a reference resistor for element 108 but also as atemperature measuring sensor for element 124 and/or for thermostatedsemiconductor body 100, the permalloy having a predeterminedrelationship between temperature and resistance. Thus semiconductor body100 can be maintained at a predetermined elevated temperature byregulating current through resistive element 128 and monitoring thetemperature of body 100 with element 124.

As disclosed, elements 108, 124, 126, and 128 are sandwiched between twolayers of dielectric such as silicon nitride, a first layer 127 alsocovering at least a portion of first surface 104, and a second layer129.

With a permalloy element 124 substantially heat sunk to semiconductorbody 100, the temperature of element 124 is substantially regulated bythe temperature of the semiconductor body. Further, since element 124 issubstantially thermally coupled to semiconductor body 100, theresistance of element 124 does not substantially vary with varyingpressure. Accordingly, a signal from element 124 may be offset by asignal from element 108, thus effectively providing a resulting signalthat under predetermined conditions of pressure will have apredetermined value. A circuit substantially as shown in FIG. 5 may beused to obtain such a result, elements 16A and 16B of FIG. 5 beingreplaced with elements 108 and 124 of the present pressure sensor, andelement 126 being put in series with either element 108 or 124 asappropriate.

The temperature versus resistance curve of permalloy elements arenonlinear. Thus, the temperature versus resistance curve of element 108will have a first predetermined slope when operating at a firstpredetermined operating temperature. The resistance of element 124 maythen be established so that, at a second predetermined temperature,e.g., typically the thermostated temperature of chip or body 100 asmeasured by element 124, the temperature versus resistance curve ofelement 124 has a slope substantially equal to the first predeterminedslope of element 108 at its operating temperature. The total effectiveresistance of element 108 or 124, as appropriate, may then be adjustedby adding series resistor means 126 in series with either element 108(this arrangement not shown) or 124 (as shown), series resistor means126 having a substantially zero thermal coefficient of resistance overthe temperature range of interest. As a result, the total effectiveresistance of the reference element may be made to be substantiallyequal at the second predetermined temperature to the total effectiveresistance of the pressure sensing element at the first predeterminedtemperature. In this manner, when the effective resistance of thereference and presssure sensing elements are substantially equal,signals through the two elements may be offset so that at apredetermined pressure the sum of the two signals will be substantiallyzero. Again this can be accomplished with a circuit substantially asshown in FIG. 5.

Although the present pressure sensor has been described in such a mannerthat it could be sensitive to changing humidity levels, that is notconsidered to be a problem for typical applications since over theuseful range of the present pressure sensor the response to changingpressure is large compared with a response to changing humidity.

When one first considers the phenomena related to the present pressuresensor, one might think that as the pressure of a gas decreases, i.e.,as the density of the gas decreases, there should be fewer molecules totransport the heat away from a heated member means comprising aresistive element. Thus, with a constant current in the resistiveelement, if there were fewer molecules, it would seem that the membermeans would always run hotter as pressure decreases. Such is the case,however, only if the mean free path length of the molecules is anappreciable fraction of the dimension between the member means and thesemiconductor body.

For pressures at which the mean free path length is short by comparisonto the distance between the member means and the semiconductor body, therate of heat transport away from the member means does not appreciablychange with the changing pressure. Although a change in pressure, e.g.,by 10%, causes the gas density to go down by a corresponding amount, themean free path and, in fact, all path lengths of every category, go upby exactly the same amount, e.g., 10%, in order to compensate. Thus, forpressures at which the mean free path length is short by comparison tothe distance between the member means and the semiconductor body, onecan make the close approximations that molecules stop when they hit, andthat the rate of heat transport away from the member means remains thesame since, although there are fewer molecules, they go 10% furtherwithout being stopped. This is a very accurate dependency orcompensation factor as long as the mean free path length of the gasmolecules is short compared to the distance between the member means andthe semiconductor body below it.

Accordingly, the present pressure sensor will not typically be sensitiveto pressures near normal atmospheric pressures, e.g., in the range ofone atmosphere to 0.1 atmosphere.

Generic Device and Processing

In terms of a particular preferred embodiment, it can be seen from thepreceding examples that a permalloy resistive element acting as a heaterand temperature sensor, in combination with the microstructuredisclosed, is a generic invention that provides the basis for sensingmany physical variables such as air flow, humidity, pressure,combustible gases and other gaseous species. In fact, any physicalquantity whose variation can cause a temperature change in a materialstructure can in principle be detected by means of a sensor based onstructure such as disclosed.

Further, member means comprising, for example, a static electric elementsuch as those disclosed can serve not only as a thermal-to-electrictransducing element for the purpose of sensing but also as, for example,an electric-to-thermal element for providing electromagnetic radiationor otherwise serving as a source of thermal energy. Of course, suchgeneric devices are not limited to having a permalloy resistive element,since any suitable thermal-to-electric or static electric element wouldsuffice. Alternate examples of elements include a pyroelectric materialsuch as a zinc oxide mono-crystalline film, a thin film thermocouplejunction, a thermister film of semiconducting material, or a metallicfilm other than permalloy with a favorable temperature coefficient ofresistance.

Therefore, described more generally than in the specific examplespreceding, and using the structure shown in FIGS. 1 through 4 asillustrative, the present invention comprises a semiconductor body 10having a depression 20 etched or otherwise formed into a first surface14 of the body. The present invention further comprises member means 32or 34 having a thermal-to-electric transducer or static electric elementsuch as 16, the member means having a predetermined configurationsuspended over depression 20, the member means being connected to firstsurface 14 at least at one location. The depression opens to firstsurface 14 around at least a portion of the predetermined configurationof the member means. The depression provides substantial physical andthermal isolation between the tranducer or element and the semiconductorbody.

Such an integrated semiconductor device can be fabricated through batchprocessing as further described below and provides an environment ofsubstantial physical and thermal isolation between the transducer orelement and the semiconductor body.

Fabrication of such a device in accordance with the present inventioncomprises providing a semiconductor body with a first surface having apredetermined orientation with respect to a crystalline structure in thesemiconductor body and applying a layer of material of which the membermeans is comprised onto the first surface. The present invention furthercomprises exposing at least one predetermined area of the first surface,the exposed surface area being bounded in part by the predeterminedconfiguration to be suspended, the predetermined configuration beingoriented so that undercutting of the predetermined configuration by ananisotropic etch will occur in a substantially minimum time.

The preferred implementation of the present invention comprisesproviding a (100) silicon wafer surface 14 which receives a layer ofsilicon nitride 12 typically 3000 angstroms thick that is deposited bystandard sputtering techniques in a low pressure gas discharge. Next, auniform layer of permalloy, typically 80 percent nickel and 20 percentiron and 800 angstroms thick, is deposited on the silicon nitride bysputtering.

Using a suitable photo mask, a photoresist, and a suitable etchant, thepermalloy element 22 comprising grid 16 and leads 24 are delineated.

A second layer 18 of silicon nitride, typically 5000 angstroms thick, isthen sputtered-deposited to provide complete step coverage of thepermalloy configuration and thus protect the resistive element and itsconnections from oxidation. (Although making the first layer of siliconnitride 3000 angstroms thick and the second layer of silicon nitride5000 angstroms thick results in a member means having non-symmetricallayers of dielectric, such lack of symmetry may be corrected by makingthe layers of equal thickness.) Openings 152 (FIGS. 10-13) are thenetched through the nitride to the (100) silicon surface in order todelineate each member means. (Note that, although the member means areshown having straight edges, such configurations could be varied by, forexample, having curved edges.)

Finally, an anisotropic etchant that does not attack the silicon nitrideis used to etch out the silicon in a controlled manner from beneath themember means (KOH plus isopropyl alcohol is a suitable etchant). Thesloping sides of the etched depression are bounded by (111) and othercrystal surfaces that are resistive to the etchant, and the bottom ofthe depression, a (100) surface, which is much less resistant to theetchant, is located a specified distance (e.g., 0.004 inch) from themember means, typically by adjusting the duration of the etch. A dopedsilicon etch stop, e.g., a boron-doped layer, may also be used tocontrol the depth of the depression, although such stops are nottypically necessary when using the present invention.

In order to obtain undercutting of the member means in a minimum amountof time, the predetermined configuration of the member means, e.g.,typically the straight edges or an axis of the member means, areoriented at a non-zero angle 154 to the (110) axis of the silicon (whilethe present invention will typically involve placing a straight membermeans edge or axis at an angle to minimize undercutting time (or permitundercutting, in the case of bridged member means), it is conceivablethat a member means could be shaped such that no straight edges areinvolved or that no axis is easily defined but that the configurationitself is still oriented to achieve, for example, minimum undercuttingtime). By making such an angle substantially 45 degrees, the membermeans will be undercut in a minimum amount of time. For example, usingthe 45 degree angle, a cantilever beam of the typical dimensionsdisclosed elsewhere herein can be undercut in about 90 minutes bycomparison to an etch time of several hours using a zero degreeorientation.

In addition to minimizing the amount of time in which the member meanswill be undercut, using a non-zero orientation permits fabrication oftwo ended bridges such as disclosed in FIG. 1. Such member means aresubstantially impossible to make with the member means edges beingoriented substantially with the (110) direction. This is because ananisotropic etch will not appreciably undercut at inside corners such as160 or at the (111) crystal planes exposed along the edges of the membermeans if the edges of member means are oriented with the (110) direction(the (110) oriented cantilevered member means taught in the prior artetch primarily along the length of the beam from the free end of thecantilever, there being little if any undercutting from the edges of thecantilever beam; this is in contrast to formation of member meansthrough the present invention which, as previously explained, results inundercutting from directions including the edges of the member means).

The 45° orientation also permits rapid rounding and smoothing of thesemiconductor end support interface with the member means, thus avoidinga stress concentration point that otherwise occurs where two (111)planes intersect beneath dielectric layer 12 (FIGS. 1-3).

It may be desirable in some device applications to connect first andsecond member means by a connecting means, thus in a sense providingfirst and second elements on a single member means. Examples of suchconnecting means are 156 shown in FIG. 10 connecting two cantileveredmember means such as shown in FIG. 2, and connecting means 158 as shownin FIG. 12 connecting two bridge-type member means of the type shown inFIG. 1. Such connecting means help to maintain uniformity of spacingand, therefore, thermal conductance between each member means and thebottom of the depression, thus contributing to the uniformity ofperformance within each type of device. For similar reasons, it may beof advantage to place two elements on a similar member means as shown byway of example in FIG. 13.

It may also be desirable in some applications, either for processing ordevice configuration, to connect the member means to the semiconductorbody at secondary locations such as locations 159 shown in FIG. 10.

Small rectangular etch holes 152 shown at the single connecting end ofthe cantilever-type member means shown in FIGS. 10 and 11 and at bothends of the bridge-type member means shown in FIGS. 12 and 13 help toprovide further undercutting and forming of the semiconductor body wherethe member means are attached. However, such holes 152 at the ends ofmember means are not necessary to satisfactory performance of thedevices.

Etch holes 152 running along side the member means shown have typicalwidths on the order of 0.002 inch to 0.005 inch for flow sensor andcombustible gas sensor applications, the width of such apertures beingon the order of 0.001 inch for typical humidity and pressure sensorapplications, the narrower humidity sensor and pressure sensor widthhelping to reduce gas flow effects.

The semiconductor bodies in FIGS. 10-13 are indicated as alternatelybeing either flow sensor configurations or combustible gas sensorconfigurations, the semiconductor bodies being alternately labeled 10 or132. Layouts for humidity and pressure sensors, e.g., in accordance withFIG. 6, would be similar, there typically being, however, only a singleelement and member means suspended over a depression in a humiditysensor.

FIGS. 10-13 also show a region 60 for integration of circuitry such asthat illustrated in FIG. 5.

As has previously been indicated, the practical effectiveness of thepresent invention for sensing by thermal means is achieved by making anair gap such as 20 (FIG. 1) underneath member means 32 or 34. The resultis that the sensing material is substantially thermally and physicallyisolated from the substrate by the air gap and is typically supported,as in the disclosed embodiments, by a rectangular area of dielectricwhich remains attached at one or both ends to the silicon substrate (aspreviously indicated, although rectangular member means are typicallyused, virtually any other configuration could be used).

For the embodiments shown, typical dimensions of member means such as 32or 34 are 0.005 inch-0.007 inch wide, 0.010-0.020 inch long, and 0.8-1.2microns thick. Typical permalloy elements such as element 16 illustratedin FIG. 4 have a thickness of approximately 800 angstroms (typically inthe range of approximately 800 angstroms to approximately 1600angstroms) with a preferred composition of 80 percent nickel and 20percent iron and a resistance value of about 1000 ohms at roomtemperature. Resistance values for varying applications would typicallybe within the range of approximately 500 ohms to approximately 2000 ohmsat room temperature, e.g. approximately 25° C. (at permalloy elementtemperatures up to approximately 400° C., resistance values increase bya factor of up to approximately 3.0). Line widths within permalloy grid16 may be approximately 6 microns having a 4 micron spacing. Depressionssuch as 20 typically have a 0.004 inch spacing between the member meansand the semiconductor body such as 10, but their spacing could easilyvary in the range of approximately 0.001 inch to approximately 0.010inch. A typical thickness of a semiconductor body or substrate such as10 is 0.008 inch. (The dimensions provided are illustrative only and arenot to be taken as limitations.)

Member means of the typical dimensions indicated have a very smallthermal heat capacity and thermal impedance that yield a thermal timeconstant of about 0.005 seconds. Consequently, a small change in heatinput results in a new thermal balance at an appreciably differenttemperature of the sensing element. This difference can yield asubstantial electrical output signal.

The strength-to-weight ratio for such a structure is very favorable, anda two-ended bridge of the above typical dimensions can withstandmechanical shock forces well in excess of 10,000 gravities. Even asingle-ended structure of these dimensions when treated as a cantileverbeam can be shown to withstand 10,000 gravities of shock.

It is a distinct advantage in many applications to heat the membermeans, e.g., 32 or 34 as shown in FIGS. 1 and 2, well above room orambient temperature to optimize its sensing performance. Typicaloperating temperatures are in the range of approximately 100° to 400° C.Using the preferred permalloy element, this can be accomplished withonly a few milliwatts of input power. Such power levels are compatiblewith integrated electronics which, as previously indicated, can befabricated on the same semiconductor body with the sensor if desired.

A standard temperature sensor in industry has an electrical impedance of100 ohms. However, for the purpose of the present invention, such asimpedance has many disadvantages. For processing purposes, it is muchmore difficult to obtain a typical impedance precision of 0.1 percentwith an impedance of 100 ohms rather than the typical 1000 ohm impedancepreferred for the preferred resistive elements in the present invention.Element failure by electromigration is also a factor in choosing atypical impedance of 1000 ohms for permalloy elements used with thepresent invention. Electromigration is a physical failure mechanismwithin a conductor caused by mass flow when currents exceed a criticallist, typically on the order of 10⁻⁶ amperes per square centimeter inpermalloy. Therefore, in order to achieve desired operating temperatureswithin permalloy elements such as 16, a relatively large impedance onthe order of 1000 ohms at room temperature (e.g., 25° C.) is desirable,the higher impedance resulting in the desired operating temperatureswithout exceeding critical current densities.

As a result, typical dimensions of the member means, e.g., 32,34, asspecified elsewhere herein must be substantially larger than the 0.001inch wide by 0.004 inch long microstructures typically reported in theprior art. The larger area of the member means typically necessary forpermalloy resistive elements compatible wih the present invention arenecessary to have sufficient surface area to mount a permalloy grid suchas 16. Thus, the preferred 45° orientation of the member means discussedelsewhere herein becomes very important from a processing timestandpoint, such an orientation resulting in a minimum processing timein creating the larger microstructures and permittng the creation ofbridged microstructures such as shown in FIG. 1.

As has been indicated, for many contemplated applications the preferredthermal-to-electric transducer or static electric element is thepermalloy resistive element previously described. When laminated withina silicon nitride member means, the permalloy element is protected fromoxidation by air and can be used as a heating element to temperatures inexcess of 400° C. Such a permalloy element has a resistance versustemperature characteristic similar to that of bulk platinum, bothpermalloy and platinum having a thermal coefficient of resistance (TCR)of about 4000 parts per million at 0° C. However, permalloy has beenfound superior to platinum for structures in accordance with the presentinvention. Although, platinum is a commonly used material fortemperature sensitive resistors, permalloy has the advantage of aresistivity about twice that of platinum. Further, in thin films,permalloy achieves maximum TCR in the thickness range of about 800 to1600 angstroms, whereas platinum films must be at least 3500 angstromsthick (permalloy achieves its maximum TCR at a thickness of about 1600angstroms, but 800 angstroms has been selected as a preferred thicknesssince resistivity is doubled and TCR is only slightly less than at 1600angstroms). Consequently, using a permalloy element 800 angstroms inthickness, the same resistance requires only one-eighth the surface areathat would be required for platinum, thus increasing the thermalefficiency of the sensing means, reducing its required area, andlowering the unit cost.

Thus, the permalloy element is both an efficient heater element and anefficient sensing element for temperature changes of microstructuressuch as those disclosed, and the combination of both heating and sensingfunctions in the same element on a substantially thermally isolatedstructure makes possible its low cost, its small thermal capacity, andits favorable sensitivity and fast response.

Further, a permalloy heater/sensor laminated into a supportinginsulating film of silicon nitride typically on the order or one micronthick provides passivation against oxygen, particularly at elevatedtemperatures, for the permalloy film. It also permits precision controlof dimensions of the member means, e.g., 32,34, because of the highresistance to process etching possessed by the silicon nitride. Inaddition, it permits deep etching to yield depressions such as 20 ofdimensions such as 0.001 inch to 0.010 inch for control of the principalthermal conductance factor.

Accordingly, using preferred embodiments of the present invention,permalloy forms both a temperature sensor and heater/radiation source incombination with the microstructure disclosed. Use of silicon nitride asa supporting and passivating material permits etching times that areneeded to achieve the desired structure. In addition, orientation inaccordance with the present invention provides undercutting in a minimumamount of time and achieves the desired structure without artificialetch stops. In addition, the use of deep anisotropic etching to controldepression depth to the 0.001 to 0.010 inch range achieves greaterthermal isolation than is possible using conventional emplacements ofthermal-to-electric or static electric elements in integratedsemiconductor devices.

The present invention is to be limited only in accordance with the scopeof the appended claims, since others skilled in the art may devise otherembodiments and methods still within the limits of the claims. Forexample, while depressions such as 20 are typically formed usingpreferential etching techniques such as those described elsewhereherein, embodiments in accordance with the present invention are notlimited to those with depressions formed by techniques described herein.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. An integrated semiconductor devicecomprising:a semiconductor body having a depression formed by ananisotropic etch applied to a first surface of the body, thesemiconductor body comprising (100) silicon having a (100) plane and a<110> direction, the first surface of the semiconductor body beingsubstantially parallel to the (100) plane; a thin film dielectric membercomprising first and second static electric elements, the member havinga predetermined configuration suspended over the depression, thepredetermined configuration being oriented at a non-zero angle to the<110> direction, the member being connected to the first surface atfirst and second substantially opposing ends so that the predeterminedconfiguration is bridged across the depression, the depression openingto the first surface around at least a portion of the predeterminedconfiguration, the depression, the opening, and the member providingsubstantial physical and thermal isolation between the first and secondelements and the semiconductor body; whereby an integrated semiconductordevice provides an environment of substantial physical and thermalisolation between the first and second elements and the semiconductorbody.
 2. The apparatus of claim 1 wherein the member comprises first andsecond member means, the first member means comprising the first staticelectric element, the second member means comprising the second staticelectric element, each member means having a predetermined configurationsuspended over the depression, each member means being connected to thefirst surface at least at one location, the depression opening to thefirst surface around at least a portion of the predeterminedconfiguration of each member means, the two member means being connectedby connecting means for connecting the first and second member meanstogether, the connecting means being suspended over the depression andphysically isolated from the semiconductor body.
 3. The apparatus ofclaim 2 wherein each static electric element comprises athermal-to-electric transducing element.
 4. The apparatus of claim 3wherein each thermal-to-electric transducing element comprises resistiveelement means.
 5. The apparatus of claim 1 wherein each static electricelement comprises a thermal-to-electric transducing element.
 6. Theapparatus of claim 5 wherein each thermal-to-electric transducingelement comprises resistive element means.
 7. A semiconductor devicecomprising:a semiconductor body having a depression formed by ananisotropic etch applied to a first surface of the body, thesemiconductor body comprising (100) silicon having a (100) plane and a<110> direction, the first surface of the semiconductor body beingsubstantially parallel to the (100) plane; a thin film dielectric membercomprising first and second thermal-to-electric transducing elements,the member having a predetermined configuration suspended over thedepression, the predetermined configuration being oriented at a non-zeroangle to the <110> direction, the member being connected to the firstsurface at first and second substantialy opposing ends so that thepredetermined configuration is bridged across the depression, thedepression opening to the first surface around at least a portion of thepredetermined configuration, the depression, the opening, and the memberproviding substantial physical and thermal isolation between the firstand second elements and the semiconductor body; whereby an integratedsemiconductor device provides an environment of substantial physical andthermal isolation between the first and second elements and thesemiconductor body.
 8. The apparatus of claim 7 wherein the membercomprises first and second member means, the first member meanscomprising the thermal-to-electric transducing element, the secondmember means comprising the second thermal-to-electric transducingelement, each member means having a predetermined configurationsuspended over the depression, each member means being connected to thefirst surface at least at one location, the depression opening to thefirst surface around at least a portion of each predeterminedconfiguration, the first and second member means being connected byconnecting means for connecting the first and second member meanstogether, the connecting means being suspended over the depression andphysically isolated from the semiconductor body.
 9. The apparatus ofclaim 8 wherein each thermal-to-electric transducing element comprisesresistive element means.
 10. The apparatus of claim 7 wherein eachthermal-to-electric transducing element comprises resistive elementmeans.
 11. A flow sensor, comprising:a semiconductor body having adepression formed by an anisotropic etch applied to a first surface ofthe body, the semiconductor body comprising (100) silicon having a (100)plane and a <110> direction, the first surface of the semiconductor bodybeing substantially parallel to the (100) plane; a thin film dielectricmember comprising first and second resistive element means, the memberhaving a predetermined configuration suspended over the depression, thepredetermined configuration being oriented at a non-zero angle to the<110> direction, the member being connected to the first surface atfirst and second substantially opposing ends so that the predeterminedconfiguration is bridged across the depression, the depression openingto the first surface around at least a portion of the predeterminedconfiguration, the depression, the opening, and the member providingsubstantial physical and thermal isolation between the first and secondresistive element means and the semiconductor body; the resistiveelement means when provided with current becoming heated, there being apredetermined relationship between the resistance of each resistiveelement means and the temperature of the respective resistive elementmeans; the first resistive element means being adapted to provide afirst signal related to the resistance of the first resistive elementmeans, the second resistive element means being adapted to provide asecond signal related to the resistance of the second resistive elementmeans, the two signals having a predetermined relationship to the rateof air flow when air flows over the member from the first resistiveelement means to the second resistive element means; whereby the firstand second signals may be offset so that at a predetermined rate of flowthe difference between the first and second signals has a predeterminedvalue.
 12. The apparatus of claim 11 wherein the member comprises firstand second member means, the first member means comprising the firstresistive element means, the second member means comprising the secondresistive element means, each member means having a predeterminedconfiguration suspended over the depression, each member means beingconnected to the first surface at least at one location, the depressionopening to the first surface around at least a portion of thepredetermined configuration of each member means, the two member meansbeing connected by connecting means for connecting the first and secondmember means together, the connecting means being suspended over thedepression and physically isolated from the semiconductor body.
 13. Theapparatus of claim 1, 2, 7 or 8 wherein the semiconductor body comprisesan integrated circuit operating in conjunction with the elements. 14.The apparatus of claim 7 or 11 wherein the predetermined configurationcomprises a substantially straight edge which is oriented at thenon-zero angle.
 15. The apparatus of claim 14 wherein the non-zero angleis substantially 45 degrees, whereby the member will be undercut themaximum efficiency.
 16. The apparatus of claim 1, 7 or 11 wherein thepredetermined configuration has an axis which is oriented at thenon-zero angle.
 17. The apparatus of claim 16 wherein the non-zero angleis substantially 45 degrees, whereby the member will be undercut withmaximum efficiency.
 18. The apparatus of claim 1, 7 or 11 wherein thenon-zero angle is substantially 45 degrees, whereby the member will beundercut with maximum efficiency.
 19. The apparatus of claim 1, 7 or 11wherein the predetermined configuration has an area greater thanapproximately 20×10⁻⁶ square inch.
 20. The apparatus of claim 1, 7 or 11wherein the member has a thickness in the range of approximately 0.8microns to approximately 1.2 microns.
 21. The apparatus of claim 1, 7 or11 wherein the depression has a depth as measured between the member andthe semiconductor body in the range of approximately 0.001 inch toapproximately 0.010 inch.
 22. The apparatus of claim 1, 7 or 11 whereinthe predetermined configuration comprises a substantially straight edgewhich is oriented at the non-zero angle.
 23. The apparatus of claim 22wherein the non-zero angle is substantially 45 degrees, whereby themember will be undercut with maximum efficiency.
 24. The apparatus ofclaim 1, 7 or 11 wherein the predetermined configuration has an axiswhich is oriented at the non-zero angle.
 25. The apparatus of claim 24wherein the non-zero angle is substantially 45 degrees, whereby themember will be undercut with maximum efficiency.
 26. The apparatus ofclaim 1, 7 or 11 wherein the non-zero angle is substantially 45 degrees,whereby the member will be undercut with maximum efficiency.
 27. Theapparatus of claim 1, 7 or 11 wherein the predetermined configurationhas an area greater than approximately 20×10⁻⁶ square inch.
 28. Theapparatus of claim 1, 7 or 11 wherein the member has a thickness in therange of approximately 0.8 microns to approximately 1.2 microns.
 29. Theapparatus of claim 1, 7 or 11 wherein the depression has a depth asmeasured between the member and the semiconductor body in the range ofapproximately 0.001 inch to approximately 0.010 inch.
 30. The apparatusof claim 11, 6 or 10 wherein each resistive element means comprises apermalloy element.
 31. The apparatus of claim 30 wherein each permalloyelement has a thickness in the range of approximately 800 angstroms toapproximately 1600 angstroms.
 32. The apparatus of claim 31 wherein eachpermalloy element has a composition of approximately 80 percent nickeland approximately 20 percent iron.
 33. The apparatus of claim 32 whereineach permalloy element has a resistance in the range of approximately500 ohms to approximately 2000 ohms at 25° C.
 34. The apparatus of claim33 wherein the member means comprises a dielectric encapsulating eachpermalloy element.
 35. The apparatus of claim 34 wherein the dielectriccomprises silicon nitride.
 36. The apparatus of claim 30 wherein eachpermalloy element has a composition of approximately 80 percent nickeland approximately 20 percent iron.
 37. The apparatus of claim 36 whereineach permalloy element has a resistance in the range of approximately500 ohms to approximately 2000 ohms at 25° C.
 38. The apparatus of claim37 wherein the member comprises a dielectric encapsulating eachpermalloy element.
 39. The apparatus of claim 38 wherein the dielectriccomprises silicon nitride.
 40. The apparatus of claim 30 wherein eachpermalloy element has a resistance in the range of approximately 500ohms to approximately 2000 ohms at 25° C.
 41. The apparatus of claim 40wherein the member comprises a dielectric encapsulating each permalloyelement.
 42. The apparatus of claim 41 wherein the dielectric comprisessilicon nitride.
 43. The apparatus of claim 30 wherein the membercomprises a dielectric encapsulating each permalloy element.
 44. Theapparatus of claim 43 wherein the dielectric comprises silicon nitride.45. The apparatus of claim 11, 6 or 10 wherein each resistive elementmeans comprises a permalloy element.
 46. The apparatus of claim 45wherein each permalloy element has a thickness in the range ofapproximately 800 angstroms to approximately 1600 angstroms.
 47. Theapparatus of claim 45 wherein each permalloy element has a compositionof approximately 80 percent nickel and approximately 20 percent iron.48. The apparatus of claim 45 wherein each permalloy element has aresistance in the range of approximately 500 ohms to approximately 2000ohms at 25° C.
 49. The apparatus of claim 45 wherein the membercomprises a dielectric encapsulating each permalloy element.
 50. Theapparatus of claim 49 wherein the dielectric comprises silicon nitride.51. The apparatus of claim 2, 8 or 12 wherein each predeterminedconfiguration comprises a substantially straight edge which is orientedat the non-zero angle.
 52. The apparatus of claim 51 wherein thenon-zero angle is substantially 45 degrees, whereby each member meanswill be undercut with maximum efficiency.
 53. The apparatus of claim 2,8 or 12 wherein each predetermined configuration has an axis which isoriented at the non-zero angle.
 54. The apparatus of claim 53 whereinthe non-zero angle is substantially 45 degrees, whereby each membermeans will be undercut with maximum efficiency.
 55. The apparatus ofclaim 2 8 or 12 wherein the non-zero angle is substantially 45 degrees,whereby each member means will be undercut with maximum efficiency. 56.The apparatus of claim 2, 8 or 12 wherein each predeterminedconfiguration has an area greater than approximately 10×10⁻⁶ squareinch.
 57. The apparatus of claim 2, 8 or 12 wherein each member meanshas a thickness in the range of approximately 0.8 microns toapproximately 1.2 microns.
 58. The apparatus of claim 2, 8 or 12 whereinthe depression has a depth as measured between each member means and thesemiconductor body in the range of approximately 0.001 inch toapproximately 0.010 inch.
 59. The apparatus of claim 12, 4 or 9 whereineach resistive element means comprises a permalloy element.
 60. Theapparatus of claim 59 wherein each permalloy element has a thickness inthe range of approximately 800 angstroms to approximately 1600angstroms.
 61. The apparatus of claim 59 wherein each permalloy elementhas a composition of approximately 80 percent nickel and approximately20 percent iron.
 62. The apparatus of claim 59 wherein each permalloyelement has a resistance in the range of approximately 500 ohms toapproximately 2000 ohms at 25° C.
 63. The apparatus of claim 59 whereineach member means comprises a dielectric encapsulating the respectivepermalloy element.
 64. The apparatus of claim 63 wherein the dielectriccomprises silicon nitride.
 65. The apparatus of claim 12, 4 or 9 whereinthe first and second member means are substantially identical, the firstand second member means comprising substantially identical first andsecond resistive element means.
 66. The apparatus of claim 11 or 12wherein the first and second resistive element means comprise a circuitfor offsetting the first and second signals.
 67. The apparatus of claim66 wherein the circuit is integrated on the semiconductor body.
 68. Theapparatus of claim 66 wherein the difference between the first andsecond signals is substantially zero at a substantially zero flow rate.69. An integrated semiconductor device comprising:a semiconductor bodyhaving a depression formed into a first surface of the body, thesemiconductor body comprising (100) silicon having a (100) plane and<110> direction, the first surface of the semiconductor body beingsubstantially parallel to the (100) plane; a thin film member comprisingfirst and second static electric elements, the member having apredetermined configuration suspended over the depression, thepredetermined configuration being oriented at a non-zero angle to the<110> direction, the member being connected to the first surface so thatthe predetermined configuration is cantilevered over the depression, thedepression opening to the first surface around at least a portion of thepredetermined configuration, the depression providing substantialphysical and thermal isolation between the first and second elements andthe semiconductor body; whereby an integrated semiconductor deviceprovides an environment of substantial physical and thermal isolationbetween the first and second element and the semiconductor body.
 70. Asemiconductor device comprising:a semiconductor body having a depressionformed into a first surface of the body, the semiconductor bodycomprising (100) silicon having a (100) plane and a <110> direction, thefirst surface of the semiconductor body being substantially parallel tothe (100) plane; a thin film member comprising first and secondthermal-to-electric transducing elements, the member having apredetermined configuration suspended over the depression, thepredetermined configuration being oriented at a non-zero angle to the<110> direction, the member being connected to the first surface so thatthe predetermined configuration is cantilevered over the depression, thedepression opening to the first surface around at least a portion of thepredetermined configuration, the depression providing substantialphysical and thermal isolation between the first and second elements andthe semiconductor body; and whereby an integrated semiconductor deviceprovides an environment of substantial physical and thermal isolationbetween the first and second elements and the semiconductor body.
 71. Aflow sensor comprising:a semiconductor body having a depression formedinto a first surface of the body, the semiconductor body comprising(100) silicon having a (100) plane and a <110> direction, the firstsurface of the semiconductor body being substantially parallel to the(100) plane; a thin film member comprising first and second resistiveelement means, the member having a predetermined configuration suspendedover the depression, the predetermined configuration being oriented at anon-zero angle to the <110> direction, the member being connected to thefirst surface so that the predetermined configuration is cantileveredover the depression, the depression opening to the first surface aroundat least a portion of the predetermined configuration, the depressionproviding substantial physical and thermal isolation between the firstand second resistive element means and the semiconductor body; theresistive element means when provided with current becoming heated,there being a predetermined relationship between the resistance of eachresistive element means and the temperature of the respective resistiveelement means; the first and second resistive element means beingadapted to provide a first signal related to the resistance of the firstresistive element means, the second resistive element means beingadapted to provide a second signal related to the resistance of thesecond resistive element means, the two signals having a predeterminedrelationship to the rate of air flow when air flows over the member fromthe first resistive element means to the second resistive element means;and whereby the first and second signals may be offset so that at apredetermined rate of flow the difference between the first and secondsignals has a predetermined value.
 72. The apparatus of claim 69, 70 or71 wherein the predetermined configuration comprises a substantiallystraight edge which is oriented at the non-zero angle.
 73. The apparatusof claim 72 wherein the non-zero angle is substantially 45 degrees,whereby the member means will be undercut with maximum efficiency. 74.The apparatus of claim 69, 70 or 71 wherein the predeterminedconfiguration has an axis which is oriented at the non-zero angle. 75.The apparatus of claim 59 wherein the non-zero angle is substantially 45degrees, whereby the member will be undercut with maximum efficiency.76. The apparatus of claim 69, 70 or 71 wherein the non-zero angle issubstantially 45 degrees, whereby the member will be undercut withmaximum efficiency.
 77. The apparatus of claim 69, 70 or 71 wherein thepredetermined configuration has an area greater than approximately20×10⁻⁶ square inch.
 78. The apparatus of claim 69, 70 or 71 wherein themember has a thickness in the range of approximately 0.8 microns toapproximately 1.2 microns.
 79. The apparatus of claim 69, 70 or 71wherein the depression has a depth as measured between the member andthe semiconductor body in the range of approximately 0.001 inch toapproximately 0.010 inch.
 80. The apparatus of claim 71 wherein eachresistive element means comprises a permalloy element.
 81. The apparatusof claim 80 wherein each permalloy element has a thickness in the rangeof approximately 800 angstroms to approximately 1600 angstroms.
 82. Theapparatus of claim 80 wherein each permalloy element has a compositionof approximately 80 percent nickel and approximately 20 percent iron.83. The apparatus of claim 80 wherein each permalloy element has aresistance in the range of approximately 500 ohms to approximately 2000ohms at 25° C.
 84. The apparatus of claim 80 wherein the membercomprises a dielectric encapsulating each permalloy element.
 85. Theapparatus of claim 84 wherein the dielectric comprises silicon nitride.86. An integrated semiconductor device comprising:a semiconductor bodyhaving a depression formed by an anisotropic etch applied to a firstsurface of the body, the semiconductor body comprising (100) siliconhaving a (100) plane and a <110> direction, the first surface of thesemiconductor body being substantially parallel to the (100) plane; athin film dielectric member comprising first and second static electricelements, the member having a predetermined configuration suspended overthe depression, the predetermined configuration being oriented at anon-zero angle to the <110> direction, the member being connected to thefirst surface at least at one location, the depression opening to thefirst surface around at least a portion of the predeterminedconfiguration, the depression, the opening, and the member providingsubstantial physical and thermal isolation between the first and secondelements and the semiconductor body; whereby an integrated semiconductordevice provides an environment of substantial physical and thermalisolation between the first and second elements and the semiconductorbody.
 87. A semiconductor device comprising:a semiconductor body havinga depression formed by an anisotropic etch applied to a first surface ofthe body, the semiconductor body comprising (100) silicon having a (100)plane and a <110> direction, the first surface of the semiconductor bodybeing substantially parallel to the (100) plane; a thin film dielectricmember comprising first and second thermal-to-electric transducingelements, the member having a predetermined configuration suspended overthe depression, the predetermined configuration being oriented at anon-zero angle to the <110> direction, the member being connected to thefirst surface at least at one location, the depression opening to thefirst surface around at least a portion of the predeterminedconfiguration, the depression, the opening, and the member providingsubstantial physical and thermal isolation between the first and secondelements and the semiconductor body; whereby an integrated semiconductordevice provides an environment of substantial physical and thermalisolation between the first and second elements and the semiconductorbody.
 88. A flow, sensor, comprising:a semiconductor body having adepression formed by an anisotropic etch applied to a first surface ofthe body, the semiconductor body comprising (100) silicon having a (100)plane and a <110> direction, the first surface of the semiconductor bodybeing substantially parallel to the (100) plane; a thin film dielectricmember comprising first and second resistive element means, the memberhaving a predetermined configuration suspended over the depression, thepredetermined configuration being oriented at a non-zero angle to the<110> direction, the member being connected to the first surface atleast at one location, the depression opening to the first surfacearound at least a portion of the predetermined configuration, thedepression, the opening, and the member providing substantial physicaland thermal isolation between the first and second resistive elementmeans and the semiconductor body; the resistive element means whenprovided with current becoming heated, there being a predeterminedrelationship between the resistance of each resistive element means andthe temperature of the respective resistive element means; the firstresistive element means being adapted to provide a first signal relatedto the resistance of the first resistive element means, the secondresistive element means being adapted to provide a second signal relatedto the resistance of the second resistive element means, the two signalshaving a predetermined relationship to the rate of air flow when airflows over the member from the first resistive element means to thesecond resistive element means; whereby the first and second signals maybe offset so that at a predetermined rate of flow the difference betweenthe first and second signals has a predetermined value.
 89. Theapparatus of claim 86, 87 or 88 wherein the predetermined configurationcomprises a substantially straight edge which is oriented at thenon-zero angle.
 90. The apparatus of claim 89 wherein the non-zero angleis substantially 45 degrees, whereby the member will be undercut withmaximum efficiency.
 91. The apparatus of claim 86, 87 or 88 wherein thepredetermined configuration has an axis which is oriented at thenon-zero angle.
 92. The apparatus of claim 91 wherein the non-zero angleis substantially 45 degrees, whereby the member means will be undercutwith maximum efficiency.
 93. The apparatus of claim 86, 87 or 88 whereinthe non-zero angle is substantially 45 degrees, whereby the member meanswill be undercut with maximum efficiency.